1. Field of the Invention
The present invention relates to electrical outlets, and more particularly to providing safety features to an electrical outlet prior to allowing the outlet to output potentially-hazardous AC power.
2. Background and Related Art
Electrical distribution points, such as power outlets, present a significant electric shock hazard in many situations. Small children, homeowners, workers on construction sites and at dockside are injured every year by preventable electrical faults. Frayed cords, metal objects inserted into an outlet, or an extension cord dropped in water may result in such a fault.
Every year thousands of people are killed or injured by accidents and fires caused by faulty electrical devices or appliances causing electrical shock. Many protective devices are being implemented to protect young children from accidentally accessing an electrical outlet and receiving injuries due to electrical shock. Inserting plastic safety plugs into a wall outlet is currently the most common way to prevent children from receiving an electric shock. Unfortunately, plastic safety plugs provide no protection unless they are reinserted each time immediately after the outlet is used. Additionally, they are ineffective if a child learns to remove them. Gated outlet plates, such as those in U.S. Pat. No. 4,970,349, also provide a measure of protection. Unfortunately, children often learn to bypass the protection provided by both plastic plug covers and gated covers. Plastic covers are also impractical on construction sites, at dockside or in many other electrical distribution systems.
Modern appliances that are more prone to cause accidents are equipped with ground fault protection. Such ground fault circuitry interrupters either interrupt the power until the electric circuit is restored to normal, for example, by manually resetting an electro-mechanical breaker. Some circuits automatically re-supply power when the circuit returns to normal. Often, such ground fault circuit interrupters are wired directly into the tool, device, or appliance, or may be molded into the cord designated for the tool or device line. Ground fault interrupters are developed to sense minute imbalances in a circuit by current leakage to ground.
Standard electrical built-in outlets, either in the home or in an industrial setting, may be also equipped with a ground fault circuit interrupter, a GFCI. Such GFCI devices provide a test function and a reset function that both work together so that a tripped GFCI cannot be reset if the GFCI circuit no longer provides ground fault protection. The test button can still be operated in the event of an open neutral condition even though the GFCI circuit is no longer powered. A built-in line load reversal feature also prevents the GFCI from resetting if the load and the conditions are mistakenly reversed. The GFCI receptacle face will be live, but there will be no power delivered to devices downstream, indicating a load reversal.
Many attempts have been made to improve the safety of electrical outlets, including the GFCI circuit discussed above. However, these attempts still have distinct disadvantages. For example, GFCI circuits leave an outlet powered until a ground fault current reaches 5-6 milliamps for at least 25 milliseconds. While this could prevent death in some instances, even this low short burst of current may be sufficient to induce potentially-fatal ventricular fibrillation. In fact, currents as low as 1 milliamp have been shown to induce ventricular fibrillation if the current passes directly through the heart. This is particularly true for small children with their smaller bodies and more fragile systems, and it is children who are most at risk due to their propensity to insert small objects, such as a paper clip, into such an outlet.
Other protective schemes are either complicated or fail to provide any more protection from short-term shocks than do the GFCI circuits discussed above. Some systems use circuits that measure loads and currents at the outlet, but such systems rely on measurements made using a full AC voltage (110 volts-240 volts, depending on the situation), and by the time such systems interrupt the measuring voltage, a lethal shock may have occurred. This is due to a failure to isolate the line voltage from the outlet's plug sockets unless a proper and turned-on load is connected to the outlet.
Other complicated systems that rely on multiple layers of protection are expensive to produce and still fail to detect many dangerous circumstances. For example, some systems rely on detecting whether a plug is inserted into the outlet before turning on power. However, this does not prevent a child from receiving a shock when a plug is properly inserted, and the child cuts the power cord with a pair of scissors or knife. Systems that include additional circuitry to limit the shock received in such a circumstance have the above-discussed limitation of failing to prevent the short-term application of potentially-lethal amounts of current, as a short time passes prior to the disconnection of power at the outlet; that is, the circuitry is unable to detect ground faults or other hazardous situations before power is supplied to the outlet and a shock hazard is created. Therefore, all current systems fail to properly protect from unwanted and potentially dangerous electric shocks.